Thanks for the reply - glad you like her name, people seem to be divided into thinking it's great or thinking it's grade one animal abuse!
Anyway, her murmur is a grade 4. When I got her (and her sister, Bob) I was all responsible, waiting until they were 12 weeks old, and booking them in for their shots and to be spayed and then found out about John's murmur. At the time they said grade 5, but it looks like one 'grade's worth' might have been the sort that they grow out of as very young kittens (when something hasn't fused or whatever). But she's left with quite a severe one, and while she hasn't had the full scans - they're just prohibitively expensive, and not covered by pet insurance - she's had a good external work up by the cardiologist and they're not happy to operate on her as she is.
So, for the moment I'm stuck with a sexually frustrated teenage cat - not to mention Bob, who has been spayed and has no idea what's going on with her sister!